Semiconductor wafers are conventionally processed in batch type furnaces. In order to obtain higher resolution, a new process, known as rapid thermal processing, is steadily replacing batch furnace processing. In this process, a semiconductor wafer is heated by a bank of quartz lamps. The temperature of the wafer must be accurately measured, because inaccurate measurements can result in inefficent or even faulty processing.
For many reasons, infrared pyrometry is a preferred method of measuring wafer temperatures, but infrared measurements to date have suffered from errors which make the results unsatisfactory. The present invention provides a means for making infrared wafer temperature measurements with sufficient accuracy for many commercial applications.
Infrared pyrometers measure the thermal radiance L at a specified wavelength [.lambda.] emitted by a hot object to calculate its temperature from the well known Planck equation EQU L[.lambda.]=Ec[.lambda.].sup.-5 [exp(d/[.lambda.]T)-1].sup.-1 [ 1]
where c and d are natural constants, T is the temperature of the target in degrees Kelvin; and E is the spectral emissivity of the target at wavelength [.lambda.]. E is a function of the surface material, surface texture, angle of observation and even target temperature itself.
The value of L can be measured very precisely by an infrared pyrometer providing an electrical output signal EQU Vo=KL [2]
where Vo is obtained when the pyrometer is operated over a small spectral range centered about wavelength [.lambda.] and K is an instrument constant. The temperature T can only be computed accurately as long as the emissivity E can be determined precisely.
It is known that the spectral emissivity E of any opaque body can be calculated from its spectral hemispherical reflectance Rh at the wavelength of operation by using the Kirchoff equation EQU E=1-Rh. [3]
Thus, this spectral emissivity can be calculated precisely when this reflectance Rh can be measured with precision.
In one known method, the target's retro-reflection is measured over a small known solid angle and the reflectance is then calculated. This is feasible when the angular reflectance pattern of the target surface is known. However, in most applications, the angular reflectance pattern is not known.
The angular reflectance of silicon wafers is not known, since the wafer surfaces have unknown angular reflectance patterns. Hence the value of the emittance is not known accurately using the above method and, therefore, the temperature cannot be measured to the accuracy desired. On the other hand, accuracy of temperature measurement is critical to successful processing of silicon wafers, wherein the wafer is placed inside of a reflecting chamber and is heated by powerful lamps within seconds to temperatures as high as 1300 degrees Centrigrade.
The present invention is directed toward apparatus which enables the emittance of wafer surfaces of arbitrary textures to be accurately measured whereby the temperature can be calculated accurately.